Corrosion inhibition in iodative dehydrogenation using molten salt systems



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GEORGE i HE\ R ATTORNEY United States Patent O 3,336,413 CORROSIONINHIBITION IN IODATIVE DE- HYDROGENATION USING MOLTEN SALT SYSTEMSHerbert L. Benson, Jr., Houston, Tex., and George S. Mill, Westport,Conn., assignors to Shell Oil Company, New York, N.Y., a corporation ofDelaware Filed Apr. 27, 1966, Ser. No. 545,707 4 Claims. (Cl. 260-680)This invention relates to the iodative dehydrogenation of organiccompounds.

Maxwell Nager, in U.S. Patent 3,080,435, issued Mar. 5, 1963, disclosesa method for dehydrogenating organic compounds which involves thefollowing steps: (l) iodative dehydrogenation of an organic compound byreacting the organic compound with elemental iodine in contact with amolten metal iodide and metal oxide or hydroxide; (2) immediatelyreacting the resulting hydrogen iodide with'the metal oxide orhydroxide; and (3) regenerating elemental iodine from metal iodide byreaction with oxygen. However, corrosion and pitting due to localizeda-ttack occurs on the surface of the reactor metal which comes incontact with the molten salt and reaction vapors.

Many of the common alloys which are available for use in otherdehydrogena-tion systems cannot be used in iodative dehydrogenationreactors, the temperatureV of the molten salt usually being greater thanabout 975 F. In these reactors, due to the presence of a very corrosivevapor phase containing, inter alia, iodine, hydrogen iodide, oxygen andsteam, corrosion in excess of 1500 mils per year may occur.

In addition to corrosion, hard, black tenacious deposits occur in theoxidation zone, which, in some cases, might lead to reactor plugging.With lithium iodide, this material has been identified as predominantlylithium ferrite, LiFeO2, the formation of which is initiated by theattack of oxygen and iodine or hydrogen iodide on steel, leading to theformation of iron oxides and subsequently to lithium ferrite.

I-t is, therefore, a principal object of the present invention toprovide an iodative dehydrogenation system wherein corrosion of thereactor is minimized. A further object is to provide a molten saltsystem for an iodative dehydrogenation reactor which minimizes theformation of lithium ferrite. Another object is to provide a method.

for the reduction of the hydrogen iodide content of the vapor phase ofan iodative dehydrogenation system.

Other objects and advantages of the invention will be apparentand-better understood from the following detailed description thereof,which will be made in part with reference to the accompanying drawing,wherein:

FIG. I is a graphic representation of the effects of lithium sulfate andsodium iodide in the molten salt system of an iodative dehydrogenationreactor on the corrosion rates of the reactor;

FIG. II is a graphic representation lshowing the effect of sodium iodidein the molten salt system of an iodative dehydrogenation reactor onhydrogen iodide loss; and

FIG. III is a graphic representation of the effect of potassium iodidein the molten salt syste-m of an iodative dehydrogenation reactor onhydrogen iodide loss.

It has now been found that corrosion can be reduced and other advantagesobtained in an iodative dehydrogenation utilizing a lithium iodide meltby providing cer- 3,336,413 Patented Aug. 15, 1 967 ice tain otherinorganic salts in the melt. In particular, it has been found thatcorrosion of -the reactor can be reduced by providing lithium sulfate,sodium iodide, or potassium iodide, or mixtures thereof, in the moltenLiI/LiOH/iodine system used in an iodative dehydrogenation reactor.

Such a reduction in corrosion rates permits reactor operation for longerperiods of -time before parts replacement becomes necessary. Besidesthis advantage there are several others, one being the improvement ofproduct yield and purity.

One of the major causes of corrosion of the reactor walls, as mentionedearlier, is extremely corrosive vapors, such as hydrogen iodide, whichcome in contact with the reactor walls. This occurs partly because ofthe incorplete reaction of the hydrogen iodide with the metal oxide orhydroxide to form the metal iodide, giving rise to hydrogen iodide slipfrom the reaction zone into the subsequent portions of the reactor. Byusing potassium or sodium iodide or lithium sulfate, or mixturesthereof, in the molten LiI/LiOH/I2 system, this hydrogen iodide slip canbe reduced substantially, not only lowering the corrosion rates, butalso increasing the efficiency of the whole system by providing a moreeffective means for hydrogen iodide capture and conversion to metaliodide. The reduction in hydrogen iodide slip in turn reduces the lossof iodine from the system, thus making the whole operation moreeconomical.

Further, there is a tendency for carbonaceous material to build up inthe reactor and become entrained in the molten salt. This coke materialtends to form a slag layer on the salt surface, and may becomesufficiently high to cause foaming in the reactor sump and in theproduct outlet line, and to increase the salt viscosity to the pointwhere the unit becomes inoperable. However, the addition of lithiumsulfate or sodium or potassium iodide, or mixtures thereof, permits thesettling-out of the carbonaceous slag by altering the physicalproperties of the salt melt, i.e., by lowering the melt density, surfacetension, viscosity and freezing point.

Among the more common alloys which are commercially available for use inthe construction of iodative dehydrogenation reactors, chrome-nickelstainless steels have exhibited the highest resistance to the iodativedehydrolgenation system. Therefore, for the purposes of describing thisinvention, these stainless steels will be considered as model metals foriodative dehydrogenation reactors. Further, for the sake of brevity andspecificity, this specification will describe the corrosion inhibitionin iodative dehydrogenation reactors with respect to the conversion ofbutane/butene to butadiene at temperatures in excess of 975 F., althoughthe invention is equally applicable to all conversions envisaged by anddisclosed in U.S. Patent 3,080,435.

Referring now to Ithe drawing, FIG. I shows graphically the `decrease incorrosion rates of an iodative dehydrogenation reactor with the additionof increasing quantities of lithium sulfate or sodium iodide to themoltenV LiI/LiOH/I2 system. As shown, the presence of the inorganic saltexerts a marked influence on the corrosion rate of the stainless steelreactor. When sodium iodide is used as the inorganic salt, 'at levels of25% by weight and greater, corrosion rates as low as to 140 mils peryear are obtained, as compared with rates of greater than 18'0'0 milsper year found in 4its absence. With lithium sulfate, at levels of 10%by weight and greater, the corrosion rates are much less than thosefound in its absence. It is apparent from FIG. I that while the ultimatecorrosion rate at high concentrations of the inorganic salt is about 100mils per year lower in the sodium iodide case um hydroxide was present.However, with a 10.9 percent by weight lithium sulfate concentration,there was a remarkable change in the appearance of the test specimens.The sample of Run 7 showed that the measured than in the lithium sulfatecase, in either case the ultimate 5 corrosion rate is caused only bygeneral metal loss, since corrosion rate with either additive 1s greaterthan ten times there was essentially no pitting of the surface of theless than that ,found m s absence' metal. In Run 8, with 10.2 percent byweight lithium sul- Table L Whlc'h f ouoyvs-1l-1ust1ates the effect, of.hthlulin fate present, a hard, black, tenacious scale formed on sulfateas a corrosion inhibitor in a molten Lil/LiOH/ 2 the Specimen, but witha c om P1 et e absence of pitting system. 10

TABLE I Run No l 1 l 2 i 3 4 i 5 i 6 l 7 i 8 l 9 i 10 l 11 l2 LOH,ercent wt.:

1 Initlial 0 3.0 3.0 5.0 0 3.1 3.0 3.1 4.9 5.0 2 9 2,3 Final 0.05 1. 61.3 2.9 0 2.3 2.6 2.2 3.8 1.4 1. 6 2 0 L' SO crcent wt.:

l2 111411121 o 0 0 0 10.0 5.1 10.9 10.2 10.0 10.1 15.0 18.4 Final 0 0 09. 7 4. 7 10. 6 9. 9 8. 0 7. s 14. 3 17. 9 Average Temperature, 1,0501,050 1,045 1,050 1,055 1,053 1,050 1,050 1,050 1,075 1,055 1,045 I2Pressure, p.s.i 7. 9 8. 3 9. 9 7A 4 7. 7 s. s 9. 0 s. 7 s. s 4 (9. 7) s.6 8. 4 Time, hr 18 19 18 18 24 18 1s 24 1s 64 18 10 Corrosion, Rate,n1/y Immersed 2 12, 600 2, 410 1,830 1, 360 1, 490 620 230 32o; 330 260 3155 3 185 a 200 Vapor 040 780 1, 100 480 590 750 740 460 600 345 460 1Corrosion rates based on weight loss of specimens. 2Specimen completelydisintegrated at end of run.

The data of Table I show that the presence of lithium sulfate in themolten LiI/LiOH/.Iz system causes a great decrease in the corrosionrates of the stainless steel reactor. At levels of 10 percent by weightand more of the lithium sulfate, the corrosion rates are approximatelyten times less than those found in the molten LiI/LiOH/I2 system withoutlithium sulfate. At levels of 3 percent by weight lithium hydroxide, andwith no lithium sulfate present, corrosion rates were found to be in therange of 1800 to 2400 mils per year (Runs 2 and 3 of Table I); and byincerasing the concentration of lithium hydroxide to percent by weightpresent in the molten salt, with still no lithium sulfate present,corrosion rates were substantially less than those found at the lowerlithium hydroxide concentrations (com-pare Run 4 with Runs 2 and 3), andeven less than those found with no lithium hydroxide present but withpercent by weight lithium sulfate present (Run 5). However, asynergistic effect is noted when Runs 7 and 5, or Runs 10 and 4, arecompared. With 3 percent by weight lithium hydroxide and 10.9 percent byweight lithium sulfate, the corrosion rate dropped to 230 mils per year,as compared with rates of about 1490 mils per year avith no lithiumhydroxide, and 10 percent by weight lithium sulfate. Run 10 demonstratesthis synergistic effect even more clearly; with 5 percent by weightlithium hydroxide, and 10.1 percent by weight lithium sulfate, corrosionrates were found to be about 155 mils per year, as compared to the rateof 1360 mils per year found in Run 4 where 5 percent by weight lithiumhydroxide, but no lithium sulfate, was present. However, since lithiumhydroxide is an integral part of the molten salt system used in iodativedehydrogenation reactors, the improvement achieved by the use of lithiumsulfate in the molten salt system is readily apparent from these data.

These data were obtained by immersing test samples of stainless steel inthe molten LiI/LiOH/Iz medium. In addition to the immersed specimens,however, identical steel specimens were positioned above the molten saltto measure the vapor phase corrosion rates. Although the data for thevapor phase measurements in Table I are much more scattered than thosefor the liquid phase measurements, a definite decrease in corrosionrates with increasing lithium sulfate concentration is found.

Of the specimens immersed in the molten Lil/ LiOH/I2 medium, several-deserve particular attention. The samples used in Runs 2 and 3 of TableI (no lithium sulfate present) showed that the high corrosion rates aredue not only to general metal loss, but also to severe localized attackresulting in extensive pitting of the metal surface. The same was foundin the case of Run 5, where no lithi- TABLE II Run No 13 i 14 15 l 16 17LiOH, percent wt.:

Initial 3.0 2. 9 3.0 3.1 3.1

Final l. 3 1.7 1.7 2. 3 1.8 NaI, percent wt.:

iti 0 13.0 19.1 25.0 31.1

Final 0 15.7 20.2 30.0 33.2 Salt temperature, F 1, 045 1,050 1, 0451,045 1,050 IzPressure, p s i a 9. 9 9. 0 9.1 8. 9 9.2 Time, hr 18 18 1818 18 Corrosion Rate, m /y Immcrsed.. 1,830 620 580 140 Vapor 1,100 540370 320 430 1 Corrosion rates based on weight loss of specimens.

The appearance of the test specimens after exposure to the LiI/LiOH/I2system containing sodium iodide is also significant. With lithiumsulfate, the samples were often found to be covered with a hard, blacktenacious scale. However, in the presence of sodium iodide, a loose,flaky substance was present on the samples, which could easily beremoved with an abrasive eraser. Although some pitting was found in thesodium iodide case, the intensity of the pitting was greatly reducedfrom that found in the molten salt system Without sodium iodide.Instead, an over-all orange-peel appearance was present.

Test runs using potassium iodide as the inorganic salt showed areduction in corrosion rates of the stainless steel specimens similar tothose found with sodium iodide. In one run, with 14 percent by weightpotassium and 3 percent by weight lithium hydroxide present, after 18hours of exposure the samples showed a corrosion rate of 156 mils peryear for the immersed specimen and 600 mils per year for the vapor phasesample.

The `appearance of the samples after exposure to a potassium iodidemodified molten LiI/LiOH/Iz system resembled that found with sodiumiodide modified system, i.e., a loose, flaky substance was present onthe samples, which could easily be removed with an abrasive eraserrevealing an over-all orange-peel effect on the underlying metalsurface.

Various combinations of sodium and potassium iodide and lithium sulfatewere found to exhibit a synergistic effect. The addition of lithiumsul-fate to melts containing potassium or sodium iodide resulted incompletely 6 dition, Table IV illustrates that lithium sulfate isbenecial in reducing hydrogen iodide slip. With about 10.5 percent byWeight lithium sulfate present, the hydrogen iodide slip is reduced from3.86 percent by weight (basis scale-free specimens, none of which showed-any sign of 5 butadiene yield) to about 2.24 percent by weight.

TABLE IV [Feed: n-Butane. Dehydrogenaton zone temperature:1,035 F.]

Run No 22 l 23 l 24 25 26 27 Ave'age Salt Composition, percent 1I 97. 294. 2 90. O 85.0 81. 4 78. 7 LiOlL.. 2. 02 2. 01 1. 95 2.18 2. 16 2. 111712003. 0.12 0. l2 0.19 0. 30 0. 37 0. 32 L12SO4 0.0 8.5 7.0 10.5 13.817. 0 Total InsolubleS 0. 44 0. 07 0. 12 0. 50 0. 54 0. 77

Hydrogen Iodide Slip, percent Wt.

(basis butadiene yield) 8. 86 3. 13 3.11 2'. 24 2. 38 2. 44

pitting. The data demonstrating this synergistic elfect are summarizedin Table III. y

TABLE III Similar results for sodium and potassium iodide are noted inFIGURES II and III, showing a reduction in the hydrogen iodide slip ineach case of up to percent 25 with 35 percent by weight KI or about 28percent by Y [Salt temperature. 1,045-1,070 F.] weight NaI m the saltmem 9 2U Tables V and VI show the effec-t of several KI-LigSO.,t Run No1 21 and NaI-Li2SO4 combinations on reduction of hydrogen mompeeent wt.:3 0 3 1 4 9 iodide slip in an iodative dehydrogenation reactor,illusiifti::311:::21:::133111111321112: 227 223 420 30 hating that asynergistic effect is `hghlih exhibited when Nar, p ercent wt.: thesesalts are used in combination.

ltial 12.9 25.2 24.9

it t TABLE v Time, br 18 18 64 Corlgnlrseh '240 32 15 35 [Feed:n-Butane. Dehydrogenation zone temperature: 1,035o F.]

Vapor 390 620 Run No 28 29 3o 31 32 l Corrosion rates based on weightloss of specimens.

In Runs 20 and 21, this synergistic effect is best il- AfgltCompositionpep nistrated. With the molten LiI/'LiOH/I2 system contain-40 .7 78.5 64.8 00.7 53.0 .0 7.2 14.7 19.9 28.0 mg 9 percent by weightlithium sulfate .and 25 percent -11 1.95 1 87 1 70 1 76 by weight sodiumiodide, the corrosion -rate was found -34 lggl li-i5 12%? .2610 to beonly 32 mils per year in a test run of 18 hou-rs duration. Extension ofthe time of exposure to 64 hours .44 2.12 1.70 1.44 1.19 Igave a valueof only 15 mils per year. With a molten 45 LiI/LiOH/Iz system containingabout .-14 percent by TABLE VI [Feed: n-Butane. Dehydrogenatlon zonetemperature: 1,035 F.]

Run No 33 34 35 36 i 37 38 l 39 Aveage Salt Composition, percent LiI31.8 74.3 69.0 64.8 53.5 52.5 6.1 12.3 17.6 23.2 23.7 34.2 LioH.. 1.781.74 1.66 1.97 1.32 1.33 L1200.. 0.30 0.33 0.35 0.50 0.44 0.72 'Lizson10.0 10.0 8.5 7.0 3.9 9.0 Hydrogen Iodide Slip, percent wt.

(basis butadiene yield) 2.50 2.32 2.13 1.72 1.28 1.41 1.20

weight 'potassium iodide and 10 percent by Weight lithium sulfate, acorrosion frate of only 31 mils per year was observed for an 18-'hourexposure.

As noted earlier, the salts of the present invention possess the utilityof Ireducing the amount of hydrogen iodide slip from the reactor, causedprimarily by incomplete reaction of the hydrogen iodide with the metaloxide or hydroxide to form the metal iodide. Referring now to FIGS. IIand III, it is seen that with increasing concentrations of sodium orpotassium iodide in the melt, the

For any of the salts to be of practical use in an iodativedehydrogenation reactor, however, it is necessary that they cause nodeleterious effects on the feed conversions and product selectivities.The data of Tables VII, VIII, and IX, Which follow, indicate that,insofar as the hydrocarbon yields are concerned, there is no deleterioustrend as the lithium iodide is diluted with lithium sulfate, sodiumiodide, or potassium iodide up to almost 50 percent by Weight. In fact,in each case, the butadiene/ hydrogen iodide slip from the reactor isreduced. In adbutene molar ratios increased.

We claim as our invention: 1. In a process for the dehydrogenation of arst hy- 5 1 231.4 ovmomoo 004661488 2. A process in accordance withclaim 1 wherein lithiurn sulfate comprises from 5 to 2O percent byweight of 35 the molten mass.

3. A process in accordance with claim 1 wherein the potassium iodidecomprises from 5 'to 3S percent by weight of the molten mass.

4. A process in accordance with claim 1 wherein sodium iodide comprisesfrom 5 to 35 percent by weight 59-752 of the molten mass.

References Cited UNITED STATES PATENTS 3,080,435 3/1963 Nager 260-67353,106,590 10/1963 Bittner 260-680 X TABLE VII [Feed: n-Butane.Dehydrogenation zone temperature: 1,035 F.]

Run No Average Sait Composition, percent Conversion, percent wt. carbonselectivity, percent wt. carbon:

TABLE VIII [Feed: n-Butane. Dehydrogenation zone temperature:1,075-1,092 E] Conversion, percent wt 8 rbon:

C5 Unsaturates. Heavy Ends (2C) Butadiene/Butene, mo1e/mole Butenes.iso-C4 Saturated. Viny1acetylene Salt Composition, percent wt.:

Run No- TABLE IX [Feed: n-Butane. Dehydrogenation zone temperature:1,000-1,050 F.] 40

Conversion, percent wt. carbon.

selectivity, percent wt. carbon (basis iced) Run Nns Salt Composition,percent wt.:

Cracked Products.-

Vinyiacetylene...

Butadiene/Butene, m0ie/m0ie 2 Figures expressed in this column are theaverages of seventeen runs.

1 Figures expressed in this column are the averages of nine runs.

1. IN A PROCESS FOR THE DEHYDROGENATION OF A FIRST HYDROCARBON TO A SECOND HYDROCARBON HAVING A HIGHER CARBON-TO-HYDROGEN RATIO, WHEREIN THE FIRST HYDROCARBON IS CONTACTED IN A REACTOR AT A TEMPERATUREIN EXCESS OF 975*F. WITH IODINE AND MOLTEN MASS COMPRISING A MIXTURE OF LITHIUM IODIDE AND LITHIUM HYDROXIDE, WHEREIN THE LITHIUM IODIDE IS THE PREDOMINANT COMPONENT OF SAID MIXTURE, THE IMPROVEMENT COMPRISING PROVIDING IN THE MOLTEN MASS AT LEAST 5 BUT LESS THAN 50 PERCENT BY WEIGHT MOLTEN LITHIUM SULFATE, POTASSIUM IODIDE OR SODIUM IODIDE, OR MIXTURES THEREOF. 